hugetlb: fix overcommit locking

proc_doulongvec_minmax() calls copy_to_user()/copy_from_user(), so we can't
hold hugetlb_lock over the call.  Use a dummy variable to store the sysctl
result, like in hugetlb_sysctl_handler(), then grab the lock to update
nr_overcommit_huge_pages.

Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Reported-by: Miles Lane <miles.lane@gmail.com>
Cc: Adam Litke <agl@us.ibm.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/hugetlb.h

@@ -33,8 +33,8 @@ int hugetlb_reserve_pages(struct inode *inode, long from, long to);
 void hugetlb_unreserve_pages(struct inode *inode, long offset, long freed);
 
 extern unsigned long max_huge_pages;
+extern unsigned long sysctl_overcommit_huge_pages;
 extern unsigned long hugepages_treat_as_movable;
-extern unsigned long nr_overcommit_huge_pages;
 extern const unsigned long hugetlb_zero, hugetlb_infinity;
 extern int sysctl_hugetlb_shm_group;
 

kernel/sysctl.c

@@ -978,8 +978,8 @@ static struct ctl_table vm_table[] = {
 	{
 		.ctl_name	= CTL_UNNUMBERED,
 		.procname	= "nr_overcommit_hugepages",
-		.data		= &nr_overcommit_huge_pages,
-		.maxlen		= sizeof(nr_overcommit_huge_pages),
+		.data		= &sysctl_overcommit_huge_pages,
+		.maxlen		= sizeof(sysctl_overcommit_huge_pages),
 		.mode		= 0644,
 		.proc_handler	= &hugetlb_overcommit_handler,
 	},

mm/hugetlb.c

@@ -24,14 +24,15 @@
 const unsigned long hugetlb_zero = 0, hugetlb_infinity = ~0UL;
 static unsigned long nr_huge_pages, free_huge_pages, resv_huge_pages;
 static unsigned long surplus_huge_pages;
+static unsigned long nr_overcommit_huge_pages;
 unsigned long max_huge_pages;
+unsigned long sysctl_overcommit_huge_pages;
 static struct list_head hugepage_freelists[MAX_NUMNODES];
 static unsigned int nr_huge_pages_node[MAX_NUMNODES];
 static unsigned int free_huge_pages_node[MAX_NUMNODES];
 static unsigned int surplus_huge_pages_node[MAX_NUMNODES];
 static gfp_t htlb_alloc_mask = GFP_HIGHUSER;
 unsigned long hugepages_treat_as_movable;
-unsigned long nr_overcommit_huge_pages;
 static int hugetlb_next_nid;
 
 /*
@@ -609,8 +610,9 @@ int hugetlb_overcommit_handler(struct ctl_table *table, int write,
 			struct file *file, void __user *buffer,
 			size_t *length, loff_t *ppos)
 {
-	spin_lock(&hugetlb_lock);
 	proc_doulongvec_minmax(table, write, file, buffer, length, ppos);
+	spin_lock(&hugetlb_lock);
+	nr_overcommit_huge_pages = sysctl_overcommit_huge_pages;
 	spin_unlock(&hugetlb_lock);
 	return 0;
 }